不能以编程方式设置NSLayoutConstraint乘数迅速..."不能分配给该前pression的结果 [英] Can't programmatically set NSLayoutConstraint multiplier in swift... "Cannot assign to the result of this expression
问题描述
我想以编程方式设置为迅速事半功倍的约束,当我设置的值,它只是给我的错误,不能分配给该前pression的结果...
I am trying to programmatically set the constraint for a multiplier in swift, and when I set the value, it just gives me the error, "Cannot assign to the result of this expression"...
我声明了一个IBOutlet的NSLayoutConstraint,然后设置倍频,同我不断的另一个原因是,它工作得很好做,但是这一次也不会接受......
I declared the NSLayoutConstraint with an IBOutlet, and then set the multiplier, same as I did with the constant for another, which works fine, but this one won't accept it...
@IBOutlet weak var clockWidthConstraint: NSLayoutConstraint!
override func updateViewConstraints() {
super.updateViewConstraints()
confirmTopConstraint.constant = 40.0
clockWidthConstraint.multiplier = 0.1 // Gives me the error!
}
任何想法?
推荐答案
是啊,这是<一个href=\"https://developer.apple.com/library/mac/documentation/AppKit/Reference/NSLayoutConstraint_Class/index.html#//apple_ref/occ/instp/NSLayoutConstraint/multiplier\"相对=nofollow>只读:
var multiplier: CGFloat { get }
您只能指定在创建时的乘数。相反,你可以在运行时更改非恒定恒
属性:
You can only specify the multiplier at creation time. In contrast, you can change the non-constant constant
property at run-time:
var constant: CGFloat
编辑:
这需要一点点努力,但要改变一成不变的属性,您必须创建一个新的约束,复制所有,但你要更改的属性。我一直在玩弄这种不同的技术,但我目前正在探索这种形式:
It takes a little effort, but to change immutable properties, you have to create a new constraint and copy all but the property that you want to change. I keep playing around with different techniques for this, but am currently exploring this form:
let constraint = self.aspectRatioConstraint.with() {
(inout c:NSLayoutConstraint.Model) in
c.multiplier = self.aspectRatio }
或以更加逼真的上下文中使用的
or used in a more realistic context:
var aspectRatio:CGFloat = 1.0 { didSet {
// remove, update, and add constraint
self.removeConstraint(self.aspectRatioConstraint)
self.aspectRatioConstraint = self.aspectRatioConstraint.with() {
(inout c:NSLayoutConstraint.Model) in
c.multiplier = self.aspectRatio }
self.addConstraint(self.aspectRatioConstraint)
self.setNeedsLayout()
}}
,其中后备code是:
Where the backing code is:
extension NSLayoutConstraint {
class Model {
init(item view1: UIView, attribute attr1: NSLayoutAttribute,
relatedBy relation: NSLayoutRelation,
toItem view2: UIView?, attribute attr2: NSLayoutAttribute,
multiplier: CGFloat, constant c: CGFloat,
priority:UILayoutPriority = 1000) {
self.firstItem = view1
self.firstAttribute = attr1
self.relation = relation
self.secondItem = view2
self.secondAttribute = attr2
self.multiplier = multiplier
self.constant = c
self.priority = priority
}
var firstItem:UIView
var firstAttribute:NSLayoutAttribute
var relation:NSLayoutRelation
var secondItem:UIView?
var secondAttribute:NSLayoutAttribute
var multiplier:CGFloat = 1.0
var constant:CGFloat = 0
var priority:UILayoutPriority = 1000
}
func priority(priority:UILayoutPriority) -> NSLayoutConstraint {
self.priority = priority;
return self
}
func with(configure:(inout Model)->()) -> NSLayoutConstraint {
// build and configure model
var m = Model(
item: self.firstItem as! UIView, attribute: self.firstAttribute,
relatedBy: self.relation,
toItem: self.secondItem as? UIView, attribute: self.secondAttribute,
multiplier: self.multiplier, constant: self.constant)
configure(&m)
// build and return contraint from model
var constraint = NSLayoutConstraint(
item: m.firstItem, attribute: m.firstAttribute,
relatedBy: m.relation,
toItem: m.secondItem, attribute: m.secondAttribute,
multiplier: m.multiplier, constant: m.constant)
return constraint
}
}
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